The present invention concerns a reversible stepping motor comprising first, second and third pole pieces which respectively comprise first, second and third pole faces defining therebetween a substantially cylindrical space, a first winding which is magnetically coupled to the first pole piece and the second pole piece, a second winding which is magnetically coupled to the first pole piece and the third pole piece, and a rotor mounted rotatably about an axis which is substantially coincident with the axis of the cylindrical space and comprising a permanent magnet having a magnetic axis substantially perpendicular to the axis of rotation.
Such a motor is described in published British patent application GB No. 2 054 978 A and may be advantageously used in an electronic timepiece for driving the time information display members, generally comprising hour and minute hands and often a second hand. The above published application also describes the method which is used to rotate the rotor of the motor in steps of 180.degree. in one direction or the other.
That method comprises supplying one of the windings with a current pulse of a first type, being of a given direction which is constant during the pulse, whenever the rotor is to rotate through one step, and at the same time supplying the other winding with a current pulse of a second type, the direction of which changes approximately in the middle of the pulse. The direction of the pulse of the first type changes for each step of the motor and so does the direction at the beginning of the pulses of the second type. The direction of rotation of the rotor depends on the winding to which the pulses of the first type are applied.
The known method involves the provision of eight power transistors in the motor actuating circuit. The eight transistors are grouped in two bridge arrangements of four transistors, one of the coils being connected into each of the bridge arrangements. The transistors take up a fairly large area on the integrated circuit which combines all the electronic circuits of the apparatus with which the motor is associated.
The motor rotates through steps of 180.degree., which means that, if for one reason or another it does not react to the current pulses which are applied to its windings at a given time, it also does not react to the pulses which are applied the moment after, as in that case the motor rotor is not in the position corresponding to the polarity of those pulses. The motor therefore loses two steps.